Liquid crystals (LCs) are currently used in a wide variety of devices, including optical devices such as information displays, including but not limited to computer screens, wristwatches, projection display devices, etc.
Generally, thermotropic liquid crystals, that is, materials with liquid crystal properties at specific temperature intervals, consist of molecules with a rod-like shape with flexible tails or with a disk-like shape. Liquid crystals simultaneously exhibit properties such as optical and electrical anisotropy of a solid crystal and fluidity of a liquid. The liquid crystal molecules have a tendency to align themselves along a common axis called the director in contrast to molecules in an isotropic (ordinary) liquid which has no intrinsic order. In the crystalline solid phase, the molecules are highly ordered and have little translational freedom. Due to the polarity of the LC-molecules, they can be ordered to point along a common director by imposing an electric or magnetic field, and the optical properties of the anisotropic liquid crystal may be manipulated accordingly.
Liquid crystals can have several molecular structures, which may be divided into three main mesophases, depending on the type of ordering within the molecular structure: Nematic, smectic and cholesteric. The nematic mesophase is the least ordered one and is characterised in that the molecules have no positional order but tend to point in the same direction. The cholesteric mesophase is very close to the nematic phase (known as the chiral nematic phase), but the director will form a helix within the material. The smectic phase is the most ordered one and is characterised in that the molecules tend to align themselves in layers, in which they may have various types of directional and transitional order. There are a total of 12 smectic mesophases identified that are different in the degree and type of order within the layers. The most important from the point of view of display application are the smectic A and smectic C. Generally, the physiochemical properties of a thermotropic liquid crystal will allow the material to exhibit several of the mesophases mentioned above within different temperature intervals.
Most liquid crystal displays are based on the ability of a liquid crystal to change the polarisation state of light. Such displays are commonly referred to as twisted nematic or super twisted nematic displays. More recently Bragg reflection in cholesteric displays has been utilised to produce reflective displays. Other liquid crystal displays are scattering and partially reflective due to steps in the index of refraction at LC domain boundaries within the display. The domains may be focal conic domains as is the case for EASL (electrically addressed smectic liquid crystal) displays or they may be liquid crystal spheres dispersed in an index-matched polymer (Polymer Dispersed Liquid Crystal or PDLC). The domains may also be encapsulated spheres with hard shells that again need to be dispersed in some index-matched medium as mentioned in U.S. Pat. No. 4,435,047. While nematic spheres are described as a single domain with a corresponding single domain wall towards the polymer matrix, smectic spheres may consist of several focal conic domains causing an improved optical scatter efficiency.
One interesting and useful characteristic for liquid crystal optical devices is that of the optical state stability. Most displays based on nematic liquid crystals, i.e., TN (twisted nematic), STN (super twisted nematic), and PDLC, are known to be monostable, that is, they will return to their original molecular configuration when the externally imposed force field such as an electric field, has been removed. The EASL display based on a smectic A liquid crystal is bistable and will thus retain its written image after the imposing force (electric field) is switched off. The same bistability is also available for some other display technologies, e.g., the cholesteric reflective display. This is a useful property for information displays which are intended to contain the same information for long periods of time.
The concept of a reflective liquid crystal display technology is particularly useful in situations with strong ambient light, e.g., outdoors in sunlight, where backlit displays effectively have no contrast, i.e., there is little difference between the perceived levels of black and white.
The concept of a bistable display is particularly useful when high resolution is required, since monostable technologies then rely on active switching elements at each picture element (pixel), which imply high production costs. The bistability may also imply low power consumption depending on (1) the energy required to switch the liquid crystal structure, (2) the time between updates, and (3) the power consumption of internal light sources.
Addition of polymer to LC has several features, as such combined materials possess properties both of LC and polymer. First of all, a polymer matrix can offer a flexible host for the liquid crystal. This means that rigid substrates such as glass etc. are not required, but flexible ones can be used such as for example indium tin oxide (ITO) coated polyesters. Secondly, the production of displays on the basis of the LC-polymer mixture is simpler and larger sizes of substrates can be used. The introduction of the polymer can also lead to new electro-optical effects (e.g. scattering in nematic polymer dispersed LCs), to obtain the stability of optical states (e.g. polymer stabilised cholesteric LC) and improved grey scale—the ability of the display to have intermediate optical states (e.g. polymer stabilised ferroelectric LC). Yet, such materials also have some disadvantages. Usually the introduction of a polymer into the LC leads to an increase in the driving voltage, i.e. voltage that is necessary to obtain a change in the optical state of the display. The degree of increase depends on the using electro-optical effect. One other disadvantage is that part of the LC volume is occupied by an optically non-active polymer, which will lead to decrease of the contrast.
The morphology of a LC-polymer mixture depends on the amount of polymer added. In the case of polymer stabilised liquid crystal (PSLC) the amount of polymer is less than 10 wt %. In this case a polymer network inside the LC is formed. When the amount of polymer is higher, the LC forms droplets inside the polymer phase and becomes a polymer dispersed liquid crystal (PDLC). The shape, size and size distribution of LC droplets determine the electro-optical properties of the material and depend on the preparation method of the PDLC as well as on physicochemical properties of the LC and the polymer.
The following methods of PSLC/PDLC preparation are well known by the art: Emulsification, phase separation and encapsulation. The first PDLC films were made by emulsifying an aqueous latex polymer solution, LC and surfactants until a dispersion was achieved. This emulsion was coated onto a substrate with a conductive layer and the water was evaporated. The soft rubbery layer obtained was laminated onto a second substrate. The original term for such material was Nematic Curvilinear Aligned Phase (NCAP) due to the curved arrangement of the nematic director within the droplets, but now this term is commonly used to point out the emulsification method that was used for preparation of PDLC.
The second widely used method for preparing PSLC/PDLC is a phase separation method. The phase separation of LC from polymer can be achieved by polymerisation of prepolymer (monomer or oligomer) in the mixture of LC and prepolymer (polymer induced phase separation or PIPS method). UV light is widely used to cure the prepolymer, but thermal polymerisation can be used for this purpose also. This method is good for PSLC preparation. It is possible to phase separate the LC and polymer from the LC/polymer solution in organic solvent during solvent evaporation (solvent induced phase separation or SIPS method). Phase separation can also occur by cooling a LC/polymer melt to room temperature (thermal induced phase separation or TIPS method).
However, information displays based on polymer dispersed liquid crystals offer several disadvantages. The main disadvantage of described methods for PDLC preparation is that the droplets of the liquid crystal will have a broad variety of shapes and sizes. This means that these methods will not give reproducible electro-optical properties of displays. Another disadvantage is that there will be diffuse interphase regions between LC droplets and polymer, mainly composed of liquid crystal, which has been swelled into the polymer matrix. These interphase regions will have different optical characteristics compared to the bulk material inside the liquid crystal droplets. This means the interfaces will not be sharp, thus reducing the quality of the clear and scatter state.